NOTE: This kind of film can get very confusing to talk about, so to make it clear, after they switch bodies and a set up reference I will mention the characters by who the personalities they are, not whose body it is.

We open in the suburban home of Dave Lockwood (Jason Bateman). It's three in the morning and the baby alarm is going off. Dave can hear the kids crying and his wife Jamie (Leslie Mann) tells him its his turn to feed the kids. Dave psyches himself up and goes into the nursery where his infant son and daughter sleep. He changes his daugther's diaper then proceeds to his son. However, in the middle of grabbing a fresh diaper, his son literally craps in his face. Not a great way to start the morning to say the least.

Dave heads to work (he's a lawyer) and talks to his associate Sabrina (Olivia Wilde) who tells him the documents for an upcoming merger are taken care of. Dave takes a long look at her as she walks away and winces. His secretary sees his attraction to her and shakes her head.

Dave gets a call from his lifelong buddy Mitch Planko (Ryan Renyolds). Dave and Mitch haven't seen each other for a while so they are going out for drinks that night. Dave is a little hesistant due to Jamie, but Mitch says he will "rape his eyes" if he bails. Dave agrees to go.

Mitch has a knock on the door. It is his father, Mitch Sr. (Alan Arkin). Mitch Sr. has come to tell his son he is getting married and his stepmother to be would like him to say a few words. Mitch is less than nice (it's implied his dad has been married several times and they have an uneasy relationship) and says he will skip this one and catch him on the next go around. His father leaves, deflated at his son's attitude.

Dave runs his kids a bath as his eldest daughter is nearby telling him about a mean girl in ballet that keeps tripping her on purpose. Dave tells her to take the high road. His daughter shows him a multicolored butterfly and tells him its her favorite. Jamie comes in asks where they want to get dinner for their weekly time to have conversations. Dave winces, and tells her he promised Mitch to go out with him. Jamie is pissed because he keeps pushing it back and their conseulor said they needed to talk at least an hour a week. Dave apologizes and asks for one last extension. Jamie sighs and says she is free Monday night. Dave agrees to that.

Dave and Mitch go to a bar and watch a baseball game. Mitch shows Dave pictures of his booty call Tatiana and how wild she is; she wants sex in numerous positions that Dave has never heard of. When Dave asks why he doesn't know these, Mitch replies, "Because you're married." Dave nods in agreement. Dave mentions his attraction to Sabrina and his inability to act on it.

Leaving the bar and walking to the car, Dave talks about how envious he is of Mitch for all the free time he has. Dave wishes he had time to learn to rollerblade, read a entire book, take a crap without stress preventing it. Mitch tells him he has a great life; a job, a wife, and kids that love him and are always there. They find a strange fountain and begin pissing in it. Their back and forth culminates with a mutual "I wish I had your life" which happens the same time as a rolling blackout. Though weirded out, they shake it off as coincidence and go home.

The next morning we see Dave get up and see freaks out to see Jamie breastfeeding one of the kids. But it is not Dave. It is Mitch in Dave's body. Mitch realizes this and promptly freaks out.

Meanwhile, in Mitch's apartment, Dave in Mitch's body wakes up to the banging of the door. It is Mitch. Mitch says he knew it and they realize that by pissing in that fountain and saying they wished they had the other's life, they switched bodies. After some freaking out, they race out to find the fountain.

Arriving at where the fountain was, Mitch and Dave find it had been uprooted to an unknown address. Going to the court clerk's office, they find out they will have to wait at least a day for the correct employee to come in and tell them where it is.

Dave is bothered because the merger deal is happening today that could guarantee a promotion and he doesn't trust Mitch since he doesn't know how to be a lawyer. Mitch meanwhile has a "acting gig" and can't miss it. The two agree to do each other's jobs, and Dave warns Mitch not to say a thing during the deal, just hand over the papers to be signed.

Mitch goes to Dave's home briefly to change then goes to the meeting. Stopping in the breakroom he steals a bunch of food. Sabrina comes in, and Mitch forgets who he is and sexually harasses her. Sabrina thinks he is having jitters and brushes it off. Mitch goes into the meeting and tries to do what Dave asked, but the company heads ask a bunch of questions he doesn't understand. He tries to improvise and causes the deal implode instead. His boss Fleming Steel (Gregory Itzin) is furious, but Mitch says it is all part of the plan. Steel tells him not to screw up whatever he is planning.

Meanwhile, Dave goes to the acting gig only to find out the role is part of a "lorno" ie. softcore porn. Dave is obviously disturbed by all this and goes through it reluctantly.

Dave and Mitch meet up and get mad at each other for screwing things up. Dave is paticulary angry at Mitch for screwing up the merger. They tell each other to "respect my life." Having enough, Dave tells Mitch they need to tell Jamie the truth. They try to, but Jamie doesn't believe them, thinking it is one big joke. So when Dave tells a story about how her vibrator short circuited and singed off a specific bit of hair down there, Jamie beats them up, thinking Dave told Mitch an intimate detail. Defeated, Dave goes back to Mitch's place.

When Mitch asked about what he should do if Jamie wants to have sex, Dave asks what day it is. "Tuesday?" Mitch says. Dave smiles and says it will be no problem.

Mitch is having dinner with Jamie and the kids but doesn't know the "dinner song" they sing. Furthermore, he is rude to Cara, Dave's daughter, hurting her feelings. Jamie thinks he should eat alone and Mitch happily agrees, taking a bunch of food with him.

Meanwhile, Dave is at Mitch's place when he hears a knock. It is Taitana, Mitch's booty call. He opens the door and Taitana walks in in nothing but a trench coat. She disrobes and reveals she is 9 months pregnant. Dave is weirded out and can't have sex with her. He winds up messing up the booty call, and Taitana tells him to never call her again.

Later when Mitch is in bed, he sees Jamie naked and thinks he will be getting lucky. Instead, Jamie goes to the bathroom in front of him, disgusting him. When Jamie tries to spoon, Mitch shoots her down and says he doesn't find her attractive. Jamie turns on her side and goes to bed mad.

Mitch is sleeping when the twins start crying. Jamie tells it is his turn but Mitch doesn't want to; she should do it because she is the mother. Jamie tells him to get out and help the kids, "or I'll fucking cut you." Mitch goes into the nursey and sees them crying their eyes out, not being able to take it.

Meanwhile, Dave is masturbating to porn when Mitch calls. Mitch needs to know how to take care of the kids, and when pressed about Taitana, Dave admits he ruined it because he couldn't deal with her being pregnant. "I could see the baby's face!" Dave exclaims. "Do you have any idea how many lamaze classes I went to in order to set that up!" Mitch screams.

Mitch tries to listen to Dave's direction but has to stop Dave's son from putting his hand in a moving blender and his daughter from playing with sharp knives. Angry and frustrated, Mitch coats the twins in a gallon of milk.

The next morning, Dave goes by the clerk office to see Victor who can tell him where the fountain is. He learns he will have to wait three days before that can be located. He later goes to his house and sees Jamie crying. Jamie is sad because her husband worked all his life since he didn't have much growing up, but it has caused him to neglect his family and her always going for the next big thing. Jamie just wants him to pay attention to her. Plus he just told her he doesn't find her attractive, and he forgot the twins at daycare. Dave is stunned about how bad he has been treating his wife.

Dave and Mitch talk. Mitch has had enough of Dave's life and is flustered that he has to wait three more days. Dave is extremely pissed off that Mitch forgot about his kids, and tells him he will have to start acting right. Dave tells him he can't quit this time. Dave takes him around and shows him how to dress and act like him in order to be a responsible person.

At the law office while Dave is there, Sabrina walks in and Mitch says some more inapporiate things. Dave picks up on that and Mitch pushes that they should the two of them should have a date on Friday which Sabrina agrees to. Mitch says Dave needs to take advantage of the time being him and getting to have sex with Sabrina is key.

Later, Mitch winds up talking to his own father and goes to lunch with him. Mitch Sr. reveals he had actually wanted Mitch at his wedding to say a few words, not his wife to be. Mitch asks why he didn't tell his son that. Mitch Sr. says partly because of how his kid acts; he is a quitter that never finishes anything. Mitch tells him that his son would think he is an asshole and leaves. He curses in the car then says "I am not a quitter!"

A montage starts in which Mitch learns to be a better lawyer, burning the midnight oil. He is also being more attentive to his kids, and handling more than the share of familial duties. Specifically, he claps during a recital where Cara gets back at the girl who had been tormenting her by pushing her down instead (something Mitch told her to do instead of taking the high road).

In the meantime, Dave realizes he doesn't have to be as rigid with his life. He spends the next couple days sleeping in, riding go karts, going to an aquarium, and learning how to rollerblade. In a telling scene while having lunch and reading a book (which he previously said he never had time for) he sees a overworked businessman and without words realizes that isn't the kind of life he wants.

Dave goes on the date with Sabrina, who tells him that it is not a date; she is only there so her boss won't fire her. Dave asks what he thought of her bosses inapporiate comments earlier. She said didn't paticulary mind; "I like to get offended, usually after work though." They have a long dinner, chatting and laughing, enjoying each other's company. Sabrina says they should do something, so they go to a tattoo parlor. Dave is getting a large tattoo on his back (not shown) that Sabrina questions but he says he wants it. Dave walks her home and Sabrina asks if he will call her. When he brings up they said this wasn't a date, Sabrina says things change. Dave smiles.

The next day, Dave gets a call about the location of the fountain.

Mitch is brushing his teeth when Jamie mentions's there anniversary. Mitch finds out he wasn't invited since Dave thought he would screw up everything. Mitch is hurt.

Dave goes to the law office and bumps into Sabrina who offers him baseball tickets for the next day. He tries to shoot her down, but Sabrina presses for it, eventually getting him to agree. Dave goes to see Mitch and says he knows where the fountain is. Mitch says he knows about it too but he wants to stay switched one more day since he worked real hard on the merger and he wants to prove he can finish things. Dave who wants to go the game with Sabrina, agrees to wait.

Mitch burns the midnight oil on the merger papers, standing up Jamie, who was meeting him for dinner and talking. Jamie calls him later and asks if he forgot something but Mitch doesn't realize he screwed up. Jamie then pours her heart out to her teenage babysitter, freaking the poor girl out.

The next day, the law firm and Mitch go to the mediation meeting. The other company gives a final offer of 625 million which they don't want to agree to. Mitch tells the CEO of the firm and Steel that they should ask for more money; specificially 725 million. Mitch says the other company wants the deal and wants to spend, and they know it because they haven't left yet. Mitch uses a metaphor of trying to have sex with a catholic schoolgirl; you just have to convince her a bit that's it's ok and she's yours.Though disturbed with the analogy, the CEO agrees and tells the other company the counteroffer and pretend to leave. The firm and Mitch barely get out the door, when the mediator tells them the company has accepted the terms. The firm is saved and Mitch is the hero, guaranteeing Mitch/Dave will be a partner in the firm.

Meanwhile, Dave goes with Sabrina to a baseball game and has a good time. It gets rained out so they go back to Mitch's place. Sabrina makes it clear she likes him and they will be having sex.

Mitch is taking Jamie and Cara to the dinner where he will become a partner in the firm. Mitch is gloating and has no clue how sad Jamie is. At the dinner he sits while Steel talks about Dave's life and how his life and marriage is so great and perfect. Jamie is near tears and Mitch realizes he can't be as great as Dave, for this is not his life. He goes to leave, as he feel he doesn't deserve the accolades bestowed upon him.

On the other side of town, Dave is disrobing Sabrina. Sabrina is talking about how why she can't be a smart successful woman and still have casual sex. Dave at first is willing to go with it, but when he sees a butterfly tattoo on Sabrina like Cara's favorite one, he realizes he doesn't want Mitch's life either, and he doesn't want to cheat on Jamie even in another person's body. He tells Sabrina he has to go home. Calling his house, he finds out he is supposed to at the partner's dinner.

Dave runs in and kisses Jamie (as Mitch mind you) to the shocked silence of the hall. Jamie realizes that she was kissed like Dave would and recognition flashes across her face. "We tried to tell you." Mitch says. Dave asks if he is ready to piss in a fountain and the two run off to set their lives right.

They drive to the address only to find out the fountain had been placed in a shopping mall. Having no choice, they begin to piss in full view of several dozen people. Mitch asks about Dave's anniversary and Dave apologizes but he no longer thinks of Mitch that way. They keep chanting they want their own lives back but it is not happening. Security is coming and running out of time they scream, "I WISH I HAD MY OLD LIFE BACK!" It finally works, causing a blackout, allowing them to escape.

Dave wakes up the next day and kisses his wife and says he is sorry about everything and he will strive to be better. He will even quit his job if she wants him too. Jamie tells him she doesnt want that, for she knows he loves his job but she wants him around more. Dave agrees to be better. He checks on his kids and listens attentively to Cara as she talks about her school day.

Mitch wakes up back in his body, happy to be back to be normal. We now see the tattoo Dave got; a huge picture of him on Mitch's back with a "I love Dave". Mitch hears a knock on the door. It is Sabrina, who forgot her purse. Sabrina apologizes for being really agressive the other day. Mitch, realizing that Dave didn't have sex with her, uses the ground work to his advantage, saying he probably acted a bit gay so he should apologize, and asks for a second shot. Mitch asks her out to breakfast and she agrees.

Mitch Sr. is getting married that following Saturday. Mitch and Dave show up as the groomsmen. Mitch Sr. is happy that his son showed up.

ONE MONTH LATER

We end on the 10th anniversary party of Dave and Jamie. Mitch gets up and says a few words about how Dave wanted to be an astronaut as a kid but he is better down here on Earth because he has a great wife in Jamie and an amazing family. The guests toast with kamakize shoots and clap for the couple. Sabrina is with Mitch now, and mentions how turned on she was by his speech. It is apparent they are now a serious couple now.

Mitch and Dave talk some more, and congratulate each other on their respective relationships, and sigh contently, realizing the value of what they have. Dave looks at Sabrina and asks Mitch, "It is weird I miss your dick?" Mitch replies, "It'd be weird if you didn't."